how can i write a letter of autorization for a school cashier telling her that am authorizing my boyfriend to pick up my paycheck instead of me
This isn't really technical writing. Just write "I authorize ___ to pick up my paycheck instead of me", on ____ (date). Giving a reason also, and a time limit. Then sign your name, and the date. It would be a good idea to try to contact payroll of your employer by telephone also, in case something else is required.
Have you considered direct deposit to your checking account? Many employers offer it.